The present invention relates to an objective lens for recording and/or reproducing for an optical information recording medium, and to an optical head and an optical pickup apparatus employing the objective lens.
Up to this time, various types of optical pickup apparatuses for conducting reproduction and recording for an optical information recording medium such as CD (compact disc) have been developed and manufactured to be made popular generally. AS objective lenses to be incorporated in these optical pickup apparatuses, there exist those each being made to be of a three-group structure to have low aberration and high NA (numerical aperture) (see Japanese Patent Application Publication No. 2003-84196).
Incidentally, if an objective lens is not limited to those for recording and reproducing for optical information, various types of objective lenses which are telecentric and of a retrofocus type are used in the fields for, for example, photographing and an endoscope (see Japanese Patent Application Publication Nos. 11-223762 and 05-307139).
However, with respect to an objective lens for an optical pickup apparatus, the telecentric optical system is not usually used, because high NA and weight reduction are required. When the objective lens is made to be telecentric, a value of NA has to be made small or the number of constituent lenses has to be increased, for correcting respective aberrations such as spherical aberration, curvature of field, distortion, coma and astigmatism. Further, even when the objective lens is made to be of a retrofocus type, it is not easy to manufacture an objective lens having less aberration with a small number of lenses.
On the other hand, when an objective lens for an optical pickup apparatus is made to be telecentric toward the image side, sharp image formation is possible in the course of information reading, which enhances pickup capability. Further, when an objective lens for an optical pickup apparatus is made to be of a retrofocus type, it is possible to enhance an NA value without increasing a diameter of an incident light flux, and thereby to enlarge a back focus corresponding to a working distance.